1. Field of the Invention
This invention relates to plate-type bed fluidized distributor structures and particularly to a fluidized bed distributor employing a modular supporting plate and baffle particularly useful in high superficial velocity applications.
Fluidization is a technique for rendering particulate matter into a suspension fluid by introduction of gas flow through the bed of matter. Particulate fluidization finds use in fossil fuel combustion furnaces as well as in the drying and curing of various materials such as cement, to name one example.
At high superficial velocities of the fluidizing fluid, e.g., at about 7 m/s (25 ft/s) near atmospheric pressure or at about 2.5 m/s (8 ft/s) at elevated pressures of about 10 atm, it is difficult to achieve particle retention and, at the same time, to maintain a low pressure drop across the fluidizing distributor. In particular, at high superficial velocities, conventional distributors will cause a pressure drop greater than 4.7 cm Hg (25 in H.sub.2 O) with a high probability of component erosion or with loss of fluidized particles through the distributor structure during turn-down.
2. Description of the Prior Art
Bubble cap or tuyere type fluidized bed distributors have the well known ability to minimize particle loss. Such distributors are unfortunately unsuited to applications utilizing high superficial gas velocities because of undesirably high pressure loss therethrough. Moreover, in elevated temperature environments, such distributors are prone to hot spot formation where lack of adequate gas circulation results in inadequate surface cooling.
Plate-type distributor designs of the prior art are documented in the literature. For example, J. F. Davidson et al., Fluidization, (Academic Press, 1971), describes plate-type distributors utilizing overlying baffles to reduce erosion, and it further describes distributor structures having gas flow patterns for minimizing particle loss. Davidson, et al. also describes realtively simple sieve-like perforate plate distributor structures which can be easily and inexpensively constructed. One known invention is described in Black et al., "Improved Air Distributor for Fluidized Beds", Chemical Engineering Progress, Volume 62, No. 3 (March 1966) page 82, which shows a distributor comprising orifices in a bottom plate having cones covering the orifices. The Black device likewise suffers from inadequate gas flow capacity and is susceptible to particle loss on turn-down.
In general, the known prior art distributor structures suffer from the shortcomings of high component erosion probability, susceptibility to hot spot formation and poor particle retention. Component erosion is caused by the collision of the fluidized particles against the surface of the distributor and its related parts which can ultimately destroy the components. Poor particle retention is the loss of particles through the distributor bed during turn-down or as a result of flow backwash of the fluidized bed or settling of the static bed. Hot spots, which are a particular problem in heated bed applications, may be caused by eddies or circulation dead spots near the surface of the distributor and related components, resulting in undesired localized heating and possible damage to structure.
A fluidized bed distrbutor is therefore needed which operates satisfactorily at high superficial velocities without the above enumerated shortcomings.